A FAST AND RELIABLE METHOD FOR OUTCROP-TO-SURFACE CORRELATION

Magnetic susceptibility (MS) and gamma-ray (GR) measurements have been used for correlation and as a proxy for sea-level variations for many years. Gamma-ray logs have been a staple in subsurface correlation for decades, but only recently have detailed outcrop gamma-ray studies been used to produce analogs for the subsurface. With the increased use of MS for correlation and as a proxy for sea-level, we believe MS may also be a valuable tool in outcrop-to-subsurface correlation. This study examines the link between magnetic susceptibility, gamma-ray, and depositional settings. Four Devonian outcrops in Clinton County Pennsylvania and well-cuttings from West Virginia were analyzed using GR and MS meters. The results were plotted on detailed stratigraphic logs. We chose three different depositional settings to test the usefulness of this approach. GR is often used to identify shale and organic rich zones in the subsurface, but this method can give false readings. MS is another method that can be used to determine the amount of clays present in the rock. Clays have magnetically susceptible minerals incorporated into their lattice and give higher MS readings than sandstones and pure carbonates. The variations of MS can be used as a proxy for sea-level (high MS during the TST/LST and low MS during HST) or deposition of fine-grained siliciclastics by rivers. The combined use of GR and MS readings can be used in organic rich shale, like the Marcellus Shale, to determine if siliciclastics input declined or organics increased. GR and MS data from the Burkett Shale demonstrates this method. The MS readings do not change but the GR in the organic rich zones increases dramatically. GR and MS data from the Tully limestone show the influence of sea-level and fluvial inputs of the fine-grained siliciclastics. During LST/TST magnetic susceptibility readings are high, but during the HST when the fine-grained siliciclastics are trapped near shore, the MS readings are low. The paleosols of the Catskill Formation exhibit high MS readings also that appear to be cyclic. These may indicate major floods and deposition of fine-grained sediments. The GR data of the paleosols show a decrease upward. This upward decrease in GR settings will give researchers and exploration geologists a fast and reliable method for correlating outcrop data with the subsurface.